1. Field of the Invention
The invention relates to a fiber-optical current transformer, comprising
a) a sensor coil with a sensor fiber for detecting a current by means of the Faraday effect,
b) a light source for generating light,
c) fiber-optical means for supplying and removing light,
d) means for polarizing the supplied light,
e) a Y splitter at one input end of the sensor fiber,
f) a mirror at a remote end of the sensor fiber, and
g) means for detecting a magneto-optically induced rotation of the direction of polarization of the light.
2. Discussion of Background
Compared with conventional measuring devices, fiber-optical sensors have considerable advantages in conjunction with electrical power applications because of their known characteristics (passive sensors, inherent direct-current isolation, non-invasive measuring probes, low weight, low requirement for space, simple integration into digital system supervision). Various prototypes are known from the literature. A more recent analysis of integrated magneto-optical sensors is supplied, for example, by the article "Magneto-optical fibre sensors for electrical industry: Analysis and performances", S. Donati, V. Annovazzi-Lodi, T. Tambosso, IEEE Proc. Vol. 135, Pt. J. No. 5, Oct. 1988, pages 372-383.
A passive fiber-optical current transformer of the type initially mentioned is known, for example, from DE 31 15 433 Al. This current transformer makes use of the Faraday effect which induces a circular birefringence in a sensor fiber. In this arrangement, the sensor fiber is carried around the current conductor in several windings. It is heavily twisted around its longitudinal axis so that the curvature-induced and the linear internal birefringence of the fiber are suppressed. The sensor fiber is operated in reflection mode. At its input end, a Y splitter having one arm each for coupling linearly polarized light in and out is located. The detection is polarimetric.
The principle of polarimetric detection is known, for example, from the publication FR 2.485.204.
The sensor coil assumes a central position in the said current transformer. It can be implemented in various manners. In each case, it is basically a matter of suppressing unwanted birefringence effects in favor of the Faraday effect. The manner in which this aim can be achieved is known, for example, from the articles
"Development of low- and high-birefringence optical fibers", D. A. Payne et al., IEEE J. of Quant. Electronics, QE-18 (1982), pages 477-488, PA0 "The rotation of the polarization in lowbirefringence optical monomode fibers due to geometrical effects", J. N. Ross, Optical and Quantum Electronics 16 (1984), pages 455-461, PA0 "Current sensors using highly birefringent bow-tie fibers", L. Li et al., Electronics Lett. 22 (1986), pages 1142-1144.
Although the directions of the approaches for implementing the fiber-optical sensors are predetermined, it is still unclear how long-time stability, operational reliability and high measuring accuracy can be ensured in the real application, that is to say under the actual, frequently rough environmental conditions (for example in a power distribution center). Especially mechanical effects (vibrations) and temperature changes can cause considerable disturbances depending on the structure of the current transformer.
It is basically desirable to use as few individual elements as possible. From this point of view, it is therefore of significance that laser diode modules have already been developed (see, for example, "Efficient highly stable laser diode module for single mode fiber employing a combination of hemispherical ended GRIN rod lens and virtual fiber", K. Kawano et al., applied optics, Vol. 28 No. 11, 1 Jun. 1989, pages 2012-2016), which are attached directly to one end of a single mode fiber (SMF) and are correspondingly distinguished by a good coupling efficiency, mechanical ruggedness and low costs.